Design and Performance of Log-Periodic Substrate Integrated WaveguideSlot Antennas

Lukui Jin, Razak M. Lee, and Ian D. Robertson

Institute of Microwaves and Photonics, University of Leeds, Leeds, LS2 9JT, United Kingdom

Abstract—Substrate integrated waveguide (SIW) technology iscombined with the classical theory of log-periodic antennas torealize SIW slot antennas with broadband performance. Basedon the orientation of the slots, two variations are compared- the transverse slot antenna and longitudinal slot antenna.Both techniques achieve much wider impedance bandwidth thana traditional uniform slotted waveguide antenna and a usefulradiation pattern is maintained over a 4 GHz bandwidth. Twoprototypes, centered at 15 and 24 GHz, have been fabricated andmeasured to verify the simulations.

Index Terms—Waveguide, log periodic antennas, slot antennas,broadband antennas.

I. INTRODUCTION

Substrate integrated waveguide (SIW) technology has re-ceived a lot of interest, in both theory and application, becauseof its high performance, ease of fabrication and potentialto be integrated with other passive and/or active microwavecomponents to create a system-on-substrate [1].

Many microwave and millimeter-wave components and cir-cuits have already been implemented based in SIW and oneof the most intriguing applications is the SIW antenna orsubstrate integrated antenna (SIA). In particular, the substrateintegrated slot antenna (SISA) has been given much attentionas it’s easy to fabricate, highly performanced and has thepotential to fulfill Antenna in Package (AiP) requirements [2].

Previously, P. Young et al. presented a simple single-slot andslot-pair antenna on SIW in 2004 and 2005, respectively[3][4].K. Wu et al. designed and implemented large SIW slotantenna arrays which characterize circular polarization and lowsidelobes [5][6]. In 2006, K. Samanta et al. realized an SIWreceiver system integrated with other microwave componentsincluding slot antennas [7].

Despite of all the merits SIW slot antennas have, one majorproblem is the narrow bandwidth, although it’s already slightlybroader than most microstrip patch antennas. Three classicalways to enhance antenna bandwidth have been given by C.Chen in [2], of which maintaining a scaling geometry in termsof wavelength is the simplest and can lead to a frequency-independent gain and radiation patterns. In [8], T. Milligangives detailed theory and design on this frequency-independentantenna, such as Archimedean spiral, conical log spiral, log-periodic dipole etc.

There are also other techniques to widen the band of slotantennas, such as using L-shaped slots [9], adding notches toslots [10]. Nevertheless, this will probably change the radiationpattern as the frequency varies and can only enhance theimpedance bandwidth.

Virtual

Apex

Half Apex Angle

α

L1L2L3

d1d2d3

R1

R2

R3

R4

Metallic Vias

SIW

Slots

g

w1w2w3x

y

z

(a) Transverse slot

Virtual

Apex

Half Apex Angle

α

R1

R2

R3

R4

Metallic Vias

SIW

Slots

w1

g1

d1

d2

d3

w2

w3

g2

g3

L1

L3

L2

x

y

z

(b) Longitudinal slot

Fig. 1. Top view of the log-periodic SIW slot array antennas (not to scale).

In this paper, based on the relationship between the slotantenna and its complementary dipole antenna, two SIW logperiodic slot antennas are presented with the aim of extendingthe bandwidth for both impedance and radiation pattern. Oneof the two antennas uses transverse slots, whilst the other useslongitudinal slots. Firstly, the basic log-periodic antenna theorywill be given, then the design of the SIW log-periodic antennaswill be described. Finally, the two techniques are comparedand a conclusion is provided.

II. THEORY

Applying the principle of the classical log-periodic dipoleantenna to the SIW slot antenna, its complementary counter-part, the log-periodic SIW slot antenna is obtained. Based onthe orientation of the slots, it could have two variations - thetransverse and longitudinal log-periodic SIW slot antenna asshown in Fig. 1.

According to [8], the slot lengths L, the distance betweenthe virtual apex and the slots, R, and the intervals between

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slots, d, can be associated, respectively, by a scaling factor, τ :

Ln = τLn-1 = τ2Ln-2 = . . . = τ n-1L1 (1)

Rn = τRn-1 = τ2Rn-2 = . . . = τ n-1R1 (2)

dn = τdn-1 = τ2dn-2 = . . . = τ n-1d1 (3)

where dn is not an independent parameter:

dn = Rn − Rn+1 = Rn(1 − τ) (4)

There are another two parameters that are key to defining theconfiguration of this slot antenna: the half apex angle α andthe spacing constant σ which are characterized as:

α = tan−1 Ln

2Rn(5)

σ =dn

2Ln(6)

together with τ , only two of them are independent.By determining the operating frequency band, the longest

and shortest slot lengths could also be roughly obtained. Afterthat, τ and σ could be selected in terms of average antennagains from empirical tables or graphs (Normally, τ and σfall into 0.8 ∼ 0.95 and 0.08 ∼ 0.15, respectively). The slotnumber n could then be calculated with the closest integer.

III. DESIGN

An SIW with a cutoff frequency of 10 GHz is first designedaccording to [11]. Here, Rogers Duroid 5880 with a permi-tivity of 2.2 is chosen as the substrate. The distance betweenvias, s, is set to be 1.8 mm with the diameter, d, 1.2 mm.The ratio of s/d is smaller than 2, which satisfies the non-leakage requirement. The effective waveguide width can thusbe calculated as 10.8 mm.

A transition between microstrip line and SIW is also de-signed and simulated using the method in [12], giving a returnloss lower than -10 dB throughout the 10 ∼ 30 GHz range.

After that, the placement and dimension of the slots areestimated. For the transverse case, τ and σ could be easilyadjusted with L confined within the width of the SIW whichis 10.8 mm, if the longest slot length, L1, is set. Here, 9 mmis chosen, which has a corresponding frequency of 16.7 GHzwhich will then be extended to K-band. For the longitudinalcase, what’s different from the transverse case is that the offsetfrom center line also needs to be scaled and there is not enoughroom in the transverse direction. In this case, another set of τand σ, τ1 and σ1, which has less radical values, is introduced.As in the transverse case, the longest slot length, L1, is fixedas 12 mm, the corresponding frequency of which is 12.5 GHz,which will then be extended to Ku band. The initial maximumslot width, w1, is set to be either 0.4 mm or 0.5 mm based onempirical knowledge.

Then, τ and σ are obtained as intermediate values, 0.9 and0.1, respectively, for both designs. τ1 and σ1 will be assumedas the same. Next is to select the right slot number to extendthe operating band to the corresponding prescribed one. n=10should be enough in this case.

(a) Transverse (b) Longitudinal

Fig. 2. Simulated bandwidth comparison between the log perdiodic andcorresponding uniform antennas.

(a) Transverse (24 ∼ 27 GHz) (b) Longitudinal (13 ∼ 16 GHz)

Fig. 3. Simulated radiation pattern of the log periodic antennas.

All the initial values can be seen in Table I. Then, the com-mercial software, HFSS, is used to accomplish the simulationand optimization.

IV. SIMULATION

Using HFSS, the configuration parameters have been ad-justed and optimized as shown in Table I. The optimumparameters can bring in maximum bandwidth, using the S11

≤ -10 dB criterion. A comparison of bandwidth is shown inFig. 2, which compares the log-periodic SIW slot antennasand the corresponding uniform antennas which are designedwith the median values of the log-periodic antennas. Also, theradiation pattern in this band has been simulated to confirm itis unchanged as shown in Fig. 3.

The bandwidth of the log-periodic slot antennas is consid-erably larger than that of corresponding uniform antennas. Forthe longitudinal design, the uniform antenna shows a multi-band performance instead of a contiguous band of operation,which makes it impractical to compare with the log-periodicdesign.

The radiation patterns of both antennas stay almost thesame over a 4 GHz frequency bandwidth, although they dosuffer some problems; the beamwidth of the transverse designis quite large and the backward radiation of the longitudinaldesign is quite high. All these problems could be resolved bymaking an array of them, which will be carried out in futurework.

V. MEASUREMENT

Two SIW log-periodic slot antenna prototypes have beenfabricated as shown in Fig. 4. Rogers Duroid 5880 is usedas the substrate and conductive epoxy is filled in the vias tomake two columns of conducting posts. The dimensions of the

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TABLE I

Parameters valuesInitial Optimized Uniform

Parameters Transverse Longitudinal Transverse Longitudinal Transverse Longitudinal

L1 (mm) 9 12 9 12 8.13 9.09w1 (mm) 0.4 0.5 0.4 0.5 0.36 0.45τ 0.9 0.9 0.96 0.94 1 1σ 0.1 0.1 0.35 0.4 0.31 0.3τ1 - 0.9 - 0.9 - 1σ1 - 0.1 - 0.15 - 0.09n 10 10 6 10 6 10

Fig. 4. Fabricated SISAs

(a) Transverse slot (b) Longitudinal slot

Fig. 5. Measurement of S parameters of two SIW log-periodic antennas

transverse design and the logitudinal one are 45 × 14 mm2

and 90 × 45 mm2, respectively. Fig. 5 shows the measuredreturn loss of the two antennas.

For SIW log-periodic transverse slot antenna, S11 has asimilar bandwidth with simulation with slightly higher lossin between the band. For the longitudinal slot antenna, theoperating band has a low loss whereas shifts towards lowerfrequencies. All these disagreements might be caused byfabrication or measurement errors, such as the conductiveepoxy doesn’t work as real metal.

VI. CONCLUSION

Two types of SIW log-periodic slot antenna, centered at15 and 24 GHz, are presented in this paper. Designed withthe theory of classical log-periodic dipole antennas, SIWlog-periodic slot antennas have greatly enhanced bandwidthcompared to traditional uniform SIW slot antennas in terms

of impedance and radiation pattern. Measurements have beenperformed to verify this.

ACKNOWLEDGMENT

The authors wish to acknowledge the financial supportof the University of Leeds FIRS Scholarship scheme. Thiswork formed part of the EPSRC project “3D Microwave &Millimetre-Wave System-on-Substrate using Sacrificial Layersfor Printed RF MEMS Components”.

REFERENCES

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[2] J. Volakis, Antenna Engineering Handbook, McGraw Hill Professional,2007.

[3] A. Farrall, and P. Young, “Integrated waveguide slot antennas”, Elec-tronics Letters, vol. 40, no. 16, pp. 974-975, August 2004.

[4] D. Stephens, P. Young and I. Robertson, “W-band Substrate integratedwaveguide slot antenna”, Electronics Letters, vol. 41, no. 4, pp. 165-167,February 2005.

[5] Z. Chen, W. Hong, Z. Kuai, J. Chen, and K. Wu, “Circularly polarizedslot array antenna based on substrate integrated waveguide”, 2008International Conference on Microwave and Millimeter Wave Techn.,vol. 3, pp. 1066-1069, 2008.

[6] J. Xu, W. Hong, P. Chen, and K. Wu, “Design and implementationof low sidelobe substrate integrated waveguide longitudinal slot arrayantennas”, IET Microw. Antennas Propag., vol. 3, no. 5, pp. 790-797,2009.

[7] K. Samanta, D. Stephens, and I. Robertson, “60 GHz multi-chip-module receiver with substrate integrated waveguide antenna and filter”,Electronics Letters, vol. 42, no. 12, pp. 701-702, June 2006.

[8] T. Milligan, Modern Antenna Design, New York: J. Wiley & Sons, 2005.[9] S.I. Latif, L. Shafai, and S.K. Sharma, “Bandwidth enhancement and size

reduction of microstrip slot antenna”, IEEE Trans. Antennas Propag.,vol. 53, no. 3, pp. 994-1003, 2005.

[10] W. Chen, and K. Ku, “Bandwidth enhancement of open slot antenna forUWB applications”, Microwave and Optical Technology Letters, vol. 50,no. 2, pp. 438-439, February 2008.

[11] F. Xu, and K. Wu, “Guided-wave and leakage characteristics of substrateintegrated waveguide”, IEEE Trans. Microw. Theory Techn., vol. 53, no.1, pp. 66-73, 2005.

[12] D. Deslandes, and K. Wu, “Integrated Microstrip and RectangularWaveguide in Planar Form”, IEEE Microw. Wireless Compon. Lett, vol.11, no. 2, pp. 68-70, 2001.

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